1. Field of the Invention
The present invention relates to a device for detecting a signal from a sensor mat composed of a plurality of sensing cells interconnected with each other in a matrix.
2. Related Art
In recent years, automobiles with air bags have been on the increase. However, some drawbacks have been pointed out concerning this air bag. In the case of an automobile with an air bag at the assist seat, some traffic accidents cause the air bag to open, although there is no person at the assist seat. A second problem is that, if a child sits on the assist seat, an open of the air bag may give the child a sudden shock. Therefore, it has been desired that the air bag at the assist seat will not open in cases where no person sits on the assist seat or a child sits on the assist seat.
To solve such undesirable situations, the present applicant has already proposed a crewmember detecting system in which a person-detecting sensor is placed. This system is therefore able to detect if a crewmember sits thereon or not, if a person on the seat is an adult or a child, and/or if a child on the seat uses a child seat or not. The person-detecting sensor used by this system is composed of a pressure sensitive sensor of which electric resistance amount changes with a change in the load to be applied. A plurality of pressure sensitive cells (that is, sensing cells) are used to detect a sat-state of a crewmember with more precision.
The more the number of sensing cells, the more the number of connecting lines with an ECU (Electric Control Unit) in charge of controlling each sensing cell and processing signals detected from each sensing cell. To overcome this inconvenience, the plural sensing cells are electrically interconnected with each in a matrix. Hence the number of connecting lines connected with the ECU has been reduced.
However, the above sensing cells interconnected in the matrix still have a problem in detecting a signal from each sensing cell.
This problem is illustrated by FIG. 1, in which both multiplexers 100 and 110 are operated to detect a signal from a sensing cell R21. A channel A2 of one multiplexer 100 and a channel B1 of the other multiplexer 110 are turned on, with the result that a flow of current is caused to pass the sensing cell R21 as shown by a solid line. The current flow enables information represented by the sensing cell R21 to be outputted to an ECU in the form of a potential divided by a pull-up resistor Rp and the sensing cell R21.
However, the flow of current is not limited to that shown by the solid line. Actually, as exemplified by a dashed line in FIG. 1, other current bypasses other sensing cells connected with the sensing cell R21on the present line occur simultaneously with the current flow through the sensing cell R21. Such bypassing currents cause a range of voltage detection to narrow, resulting in that it is difficult to detect a signal from each sensing cell with precision.
An object of the present invention is to provide, with due consideration of the above problem, a detecting device capable of, with more precision, detecting a signal from a sensor composed of a plurality of sensing cells by preventing current from bypassing other sensing cells other than a specified sensing cell to be detected.
In order to realize the above object, the present invention provides a detecting device (1) for detecting a signal from a sensing mat (2). In the sensing mat (2), a plurality of sensing cells (R) are electrically interconnected to each other in a matrix by a plurality of line paths (2a). Each sensing cell (R) has resistance of which amount changes with a change in a load to be applied. The detecting device (1) comprises a detecting circuit (Rp) and voltage applying means (5). The detecting circuit (Rp) detects the signal through a conversion of changes in the amount of the resistance to changes in voltage. When detecting the signal of a sensing cell (R11) connected to a specified line path (2a) of the sensing mat (2), the voltage applying means (5) applies a particular voltage to the remaining one or more line paths (2a).
Preferably, the particular voltage is voltage of which potential is the same as a voltage detected from the selected sensing cell (R11). Alternatively the particular voltage is voltage of which potential is the same as a voltage applied to the selected line path (2a).
Accordingly, for detecting a signal from each sensing cell (R) connected to each line path (2a) of the sensing mat (2), current is avoided from bypassing other line paths irrelevant to the signal detection of a specified senescing cell.
It is preferred that the voltage applying means (5) includes a buffer circuit (5) having an output terminal (5b) for outputting the voltage of which potential is the same as the voltage detected from the selected sensing cell (R1l). In this case, it is still preferred that the device comprises a switching element (3) having a plurality of first terminals (3a) each connected to each line path (2a) of the sensing mat (2) and a plurality of second terminals (A) selectively connecting the plurality of first terminals (3a) to the output terminal (5b) of the buffer circuit (5). For example, the buffer circuit (5) is one in number. The switching element (3) may include a plurality of third terminals (B) selectively connecting the plurality of first terminals (3a) to the detecting circuit (Rp). Preferably, each of the first, second and third terminals (3a, A, B) arranged in the switching element (3) is equal in number to the line paths (2a) of the sensing mat (2).
Accordingly, thanks to a combination of the buffer circuit and the multiplexer, only one buffer circuit is enough for applying the voltage to avoid the current bypass to the other line paths irrelevant to the signal detection. In other words, it is unnecessary to arrange the buffer circuit for each line path of the sensing mat.